1. Technical Field
The technical field relates to an electrolyte mixture for an electrolytic capacitor, a composition for conductive polymer synthesis, and a conductive polymer solid electrolytic capacitor.
2. Background
Improving the electrolyte conductivity has long been one of the major topics in the development of an electrolytic capacitor. The electrolyte with a high conductivity can reduce the equivalent series resistance (ESR) of the electrolytic capacitor, so as to provide high-frequency low impedance and high reliability. A conductive polymer has a higher conductivity than an aqueous electrolyte or a solid organic semiconductor complex salt (e.g. tetracyanoquinodimethane (TCNQ) complex salt) used for conventional capacitors, and exhibits an adequate insulating property at high temperature. Besides, the conductive polymer is safe and free of explosion of a liquid capacitor caused by liquid evaporation. Therefore, such conductive polymer has become the mainstream of the solid electrolyte for existing electrolytic capacitors.
The conductive polymer serving as a solid electrolyte was first proposed in U.S. Pat. No. 4,803,596. In the forming method thereof, an anode foil is dipped in a solution of a monomer and an oxidant, and a polymerization is carried out at an adequate temperature. However, the monomer and the oxidant are reacted so quickly that the conductive polymer can not cover the electrode uniformly.
Low yield and high impedance are often observed in the fabrication of a conductive polymer solid electrolytic capacitor. An inhibiting agent such as imidazole or a derivative thereof is added to reduce the reaction rate and improve the properties of the solid electrolytic capacitor. However, such method can be only applied to low/medium voltage solid electrolytic capacitors. The withstanding voltage of high voltage (>50 V) solid electrolytic capacitors cannot be effectively increased with such method, resulting in low yield production.
In recent years, conductive polymer high voltage capacitors have been widely applied in vehicles. The reliability of the conductive polymer high voltage capacitors is poor for some reasons. First, the conductive polymer has poor film properties and is highly brittle. In long-term use, the vibration from the environment causes deformation of the conductive polymer and breakdown of the oxide layer. Besides, the conductive polymer has poor film forming property and therefore is not able to completely cover the surface of the dielectric layer of the capacitor, and thus, the rate of capacitance withdrawing of the solid electrolytic capacitor is reduced. Moreover, when the material of the dielectric layer is crystalline oxide or aluminium oxide formed by anodization, the aluminium oxide layer inside the dielectric layer is inhomogeneous. Therefore, cracks exist at grain boundaries in the dielectric layer so as to cause a leakage current.
In view of the above, solid electrolytic capacitors can overcome the disadvantages of liquid aluminium electrolytic capacitors. However, upon the practical use and test, many drawbacks are still found in the said techniques. A solid electrolyte is used instead of a liquid electrolyte so as to eliminate the poor temperature properties and long-term instability of the liquid. However, a greater leakage current occurs when the solid electrolytic capacitor is operated under high load or high temperature. A short circuit caused by over leakage current is observed if the film forming property cannot be improved. From the test results, the highest withstanding voltage of conventional solid electrolytic capacitors is less than 50 V. Accordingly, attention has been drawn to how to increase the withstanding voltage of a solid electrolytic capacitor.